This invention relates in general to RF receivers and is particularly directed to an RF amplifier and IF automatic gain control (AGC) system for a UHF receiver.
A conventional television receiver includes a radio frequency (RF) section and an intermediate frequency (IF) section. The RF section includes RF filters which are turned to coarsely filter a band of frequencies centered about a selected channel. The output of the RF filter is provided to the input of an RF amplifier which typically has a gain of at least 20 to 30 db. The output of the RF amplifier couples to one input of a mixer, while a second input of the mixer receives mixing signals of a selectable frequency from a local oscillator in generating a heterodyned lower, intermediate frequency (IF) of approximately 45 MHz. The output of the mixer is filtered and amplified for processing in an IF section for tuning to a selected channel.
Tuning over the UHF band from 470 MHz (channel 14) to 890 MHz (channel 83) presents special difficulties because of this large frequency range. Prior art UHF tuners have suffered from limitations in signal handling capability over such a large bandwidth. Some of the shortcomings have been overcome by the use of metal-oxide-semiconductor (MOS) field-effect transistors (FET's) in the front end and tuning stages of the television receiver. MOSFET's offer a high input impedance of from 10.sup.9 to 10.sup.15 ohms for improved impedance matching with the antenna and RF filters in the front end of the receiver. In addition, the gain-bandwidth figure of a MOSFET can easily be made greater than 100 MHz. A further advantage in many applications is the thermal stability of MOSFET's and the lack of thermal runaway. In contrast to bipolar transistors, their current decreases with increasing temperature.
Because of the various inherent advantages in MOSFET operation, they are finding increased use in the front end of RF receivers. For example, U.S. Pat. No. 4,380,828 to Moon discloses a UHF television receiver tuner which includes a MOSFET mixer stage. MOSFET's have also been used in the received signal amplification stage in a television receiver as shown in FIG. 1.
FIG. 1 shows the use of a MOSFET 12 in the front end of a UHF receiver 10. The received signal is provided to the G1 gate of MOSFET 12 via antenna 14. The resistance of the current path from the MOSFET's source (S) to drain (D) electrodes is modulated by the voltage applied to the G2 gate of MOSFET 12. A V.sub.S biasing voltage is applied to the source electrode of MOSFET 12 via a voltage dividing network (not shown). The amplified output of MOSFET 12 is provided from the drain to mixer circuit 24 via AC coupling capacitor 22. Also provided to mixer circuit 24 is a reference frequency output from local oscillator 26 for generating a 45 MHz IF signal which is provided via blocking diode 28 and two-position U/V switch 37 to an IF amplifier 30 and thence to an IF section 38. Provision is also made for providing a VHF IF signal via U/V switch 37 and VHF RF amplifier and mixer stages 36, 33 to IF amplifier 30 when it is desired to tune to a VHF signal received by VHF antenna 35. A choke 32 is coupled between the cathode of diode 28 and the drain of MOSFET 12 and is connected to a V.sub.IN voltage source which drives the MOSFET 12. Choke 32 isolates the AC signal in the UHF receiver front end 10 from the V.sub.IN supply. Resistor 61 isolates the cathode of blocking diode 28 from V.sub.IN and from the output of MOSFET 12. The output of the IF amplifier 30 is provided to an automatic gain control (AGC) system 34 which, in turn, provides a MOSFET control input to the G2 gate thereof. Thus, the amplitude of the IF signal provided from the receiver front end to the IF section 38 is controlled in a feedback arrangement by means of the voltage applied to the G2 electrode of MOSFET 12 from AGC system 34.
Ideally, the AGC-controlled input to the control gate of MOSFET 12 ensures a relatively constant output from the IF amplifier to the IF section. However, because of variations in the signal processing characteristics of the UHF receiver front end primarily due to the wide frequency range of the UHF band, variations in IF signal amplitude are encountered. These variations in IF signal level may cause the video amplifiers to become overloaded in the case of a strong incoming signal, resulting in cross modulation and clipping of the synchronizing components of the received video signal. On the other hand, a weak incoming signal may cause the output of the various video amplifiers to be too low to provide proper picture reproduction. Variation in IF signal level is caused not only by variations in operating characteristics of the UHF receiver front end, but also by limitations in the operation of the AGC circuit.
The present invention is intended to overcome the limitations of the prior art by providing a UHF RF amplifier/IF AGC system having constant gain over the entire UHF bandwidth which provides isolation between the RF and IF signals for reduced cross-modulation and intermodulation interference. In addition, the present invention is capable of operating at reduced voltage levels for improved thermal operation and increased reliability.